Magnetic recording techniques have been widely used in various fields including video, audio and computer uses, since magnetic recording techniques have characteristics which are not found in other recording systems, such that a recording medium can be used repeatedly, signals can be easily electronized and a system by combination with peripheral equipments can be constructed, and signals can be easily modified.
And further improvements of recording density, reliability and durability of the magnetic recording medium have always been desired to respond to requirements, such as the miniaturization of apparatus, the improvement of the quality of recording and reproducing signals, long time recording, and the increase of recording capacity.
In recent years, in the magnetic recording and reproducing system for recording and reproducing computer data, a system into which a thin film magnetic head is incorporated has been put to practical use. Since thin film magnetic heads are easy to be miniaturized or processed to multitrack heads, multitrack fixed heads of thin film magnetic heads are largely used in systems using magnetic tapes as recording media. By the use of thin film magnetic heads, improvements of track density and recording efficiency are realized by virtue of the miniaturization, which lead to the realization of high density recording, as well as the improvement of data transfer speed by the realization of multitrack. Thin film magnetic heads can be classified broadly into two categories of an induction type head of responding to time variation of flux of magnetic induction, and a magneto-resistance type (MR) head utilizing a magneto-resistance effect responding to the size of magnetic flux. Since an induction type head is of planar structure, the number of winding of coil is less, hence it is difficult to make magnetomotive force high, so that sufficient reproduction output cannot be obtained. Therefore, an MR head capable of easily obtaining high reproduction output is used for reproduction, on the other hand, an induction type head is used for recording. These magnetic heads are generally incorporated into a system as integral. In such a magnetic recording system, a linear recording method capable of realizing higher data transfer speed is adopted.
On the other hand, the development of a rotating drum-mounting MR head suitable for a helical scan system tape recording apparatus is being advanced for realizing tape recording of a high transfer speed of high capacity magnetic data with a small-sized cassette. In audio and video uses, to put to practical use a digital recording system capable of realizing the improvement of sound and image qualities, and to answer to the development of a picture recording system corresponding to high vision TV, a demand has been increased for a magnetic recording medium which can record and reproduce further shorter wavelength signals than conventional systems and reliability and durability are secured even if the relative speed of a head and a medium becomes large.
For increasing the recording density of a coating type magnetic recording medium, a variety of methods have been discussed and suggested from the viewpoints of, e.g., by use of iron or a magnetic alloy powder comprising iron as a main component in place of conventionally used magnetic iron oxide powders, the improvement of magnetic powders such as fining of magnetic powders and the improvement of the packing density and the orientation property, improving the magnetic characteristics of a magnetic layer, improving the dispersibility of ferromagnetic powders, and improving the surface property of a magnetic layer.
For instance, methods of using ferromagnetic metal powders and hexagonal ferrite powders as ferromagnetic powders for increasing magnetic characteristics are disclosed in Japanese Patent (Application) Laid-Open Nos. 122623/1983, 74137/1986, Japanese Patent Publication Nos. 49656/1987, 50323/1985, U.S. Pat. Nos. 4,629,653, 4,666,770 and 4,543,198.
Japanese Patent Laid-Open No. 18961/1989 discloses a ferromagnetic powder having a specific surface area of from 30 to 55 m2/g, a coercive force of 1,300 Oe or more, and a saturation magnetization (amount) of 120 emu/g or more produced from a magnetic metal powder having a long axis diameter of from 0.05 to 0.2 μm and an axis ratio of from 4 to 8, and suggests to provide fine metal powders having a small specific surface area. There are disclosed in Japanese Patent Laid-Open Nos. 11300/1985 and 21307/1985 producing methods of a fine α-iron hydroxide acicular crystal suitable for a ferromagnetic powder, in particular, a ferromagnetic metal powder, and Japanese Patent Laid-Open No. 21307/1985 discloses that a ferromagnetic metal powder having a coercive force (Hc) of from 1,450 to 1,600 Oe, and a saturation magnetization (σs) of from 142 to 155 emu/g can be produced from goethite having a long axis length of from 0.12 to 0.25 μm and an axis ratio of from 6 to 8. Japanese Patent Laid-Open No. 91684/1997 proposes to use ferromagnetic metal particles wherein ferromagnetic metal particles having an average long axis diameter of from 0.05 to 0.12 μm and an acicular ratio of 8 or more account for 5.0% or less of the entire ferromagnetic metal particles, or to use ferromagnetic metal particles wherein crystallites having an acicular ratio of 4 or more which constitute the particles account for 17.0% or less of the entire ferromagnetic metal particles. However, when particles having a small acicular ratio are mixed, a ferromagnetic powder having high Hc is difficult to obtain, and S/N and overwriting characteristics are insufficient.
Further, a hematite nucleic crystal, iron hydroxide, monodispersed hematite particles of a spindle configuration using specific ions, and an extremely fine ferromagnetic powder obtained by reducing the hematite particles are disclosed in Japanese Patent Laid-Open Nos. 340426/1994 and 109122/1995.
For increasing the dispersibility of a ferromagnetic powder, it is proposed to use various kinds of surfactants (e.g., in Japanese Patent Laid-Open Nos. 156606/1977, 15803/1978 and 116114/1978), and a variety of reactive coupling agents (in Japanese Patent Laid-Open Nos. 59608/1974, 58135/1981 and Japanese Patent Publication No. 28489/1987).
Further, Japanese Patent Laid-Open No. 239819/1989 proposes a magnetic powder obtained by adhering in order a boron compound, an aluminum compound, or an aluminum compound and a silicon compound on the particle surface of a magnetic iron oxide to improve magnetic characteristics and dispersibility of the magnetic powder. Further, Japanese Patent Laid-Open No. 22224/1995 discloses a ferromagnetic metal powder containing 0.05 mass % or less of the elements belonging to Group Ia of the Periodic Table and, if necessary, from 0.1 to 30 atomic % of aluminum based on the total amount of the metal elements, and from 0.1 to 10 atomic % of rare earth elements based on the total amount of the metal elements, and 0.1 mass % or less of residues of the elements belonging to Group IIa of the Periodic Table, to thereby obtain a high density magnetic recording medium excellent in storage stability and magnetic characteristics.
Moreover, for improving the surface property of a magnetic layer, a method of improving surface treating method of a magnetic layer after coating and drying is suggested (e.g., disclosed in Japanese Patent Publication No. 44725/1985).
On the other hand, for achieving high recording density of a magnetic recording medium, to make shortwave of signals to be used has been aggressively advanced. However, when the length of the region to record signals becomes the size comparable with the size of the magnetic material used, clear magnetization transition state cannot be formed, as a result recording becomes substantially unfeasible. For that reason, it is necessary to develop a magnetic material having sufficiently minute particle size relative to the minimum wavelength used, therefore, fining of a magnetic material has been pointed out for a long time.
In a metal powder for magnetic recording, an aimed coercive force is obtained by making a particle configuration acicular and providing configuration anisotropy. It is well known in the art that the surface roughness of the medium obtained by fining a ferromagnetic metal powder must be made small for high density recording. However, when a metal powder for magnetic recording is made fine, the acicular ratio of the metal powder reduces with fining and the desired coercive force cannot be obtained. In recent years, a DVC system of recording video signals by digitization has been suggested and a metal evaporation tape (an ME tape) of high performance and a metal powder (coating) tape (an MP tape) of high performance are used for that uses. Since the coercive force of MP tapes for use in DVC is 2,000 Oe or more, ferromagnetic metal powders are required to have a high coercive force, to be fine, and have excellent particle size distribution. Further, tapes are in many cases used in the form of recording methods of overwriting magnetic signals, they are desired to have good overwriting characteristics.
The present inventors have proposed a ferromagnetic metal powder suitable for a DVC system and a magnetic recording medium using the same (Japanese Patent Laid-Open No. 326035/1995). This invention is to provide a magnetic recording medium in which a magnetic layer is restrained to have a coercive force of from 2,000 to 3,000 Oe, a layer thickness of from 0.05 to 0.3 μm and surface roughness of from 1 to 3 nm, and the magnetic flux revolution factor is specified.
Further, as the magnetic recording medium for use in a magnetic recording system having incorporated thin film magnetic heads, a magnetic recording medium comprising a nonmagnetic support having provided thereon a lower nonmagnetic layer containing an inorganic nonmagnetic powder dispersed in a binder, and an upper magnetic layer containing a ferromagnetic metal powder dispersed in a binder on the nonmagnetic layer is proposed (Japanese Patent Laid-Open No. 227517/1996). Since output reduction due to thickness loss can be inhibited and high recording density can be achieved by thinning an upper magnetic layer as above, data storage of higher capacity becomes possible as compared with a magnetic recording medium having a magnetic layer of single layer structure. The patent discloses that the layer thickness of the upper magnetic layer is from 0.05 to 1.0 μm, preferably from 0. 05 to 0.8 μm. Specifically, a magnetic recording medium for computer data recording comprising a polyethylene terephthalate support having a thickness of 10 μm having, on one side of the support in order, a nonmagnetic layer having a thickness of 2.7 μm and a magnetic layer having a thickness of 0.3 μm containing a ferromagnetic metal powder having a coercive force (Hc) of 1,800 Oe provided is disclosed.
In a magnetic recording system having incorporated MR heads, there are problems in the adaptability of the MR heads to the magnetic recording medium for use in the system.
That is, when a magnetic recording medium having a comparatively thick magnetic layer (0.3 μm) is used as the magnetic recording medium, since the magnetic flux of the magnetic layer becomes high, MR heads are saturated due to the overbalance of reproduction output, the reproduction waveform is deformed, as a result, a sufficiently high S/N value cannot be obtained, and an error rate is liable to increase. It was also found that recording and reproduction waveform (isolated reproduction revolution waveform) is generally preferably sharper (the half value width of the waveform is small) for achieving high recording density, however, in a magnetic recording medium having a relatively thick magnetic layer, the half value width of recording and reproduction waveform becomes great, so that sufficiently high recording density cannot be obtained.
On the other hand, when a magnetic recording medium having a very thin magnetic layer (0.03 μm) is used, the recording and reproduction waveform is deformed, as a result, a sufficiently high S/N value cannot be obtained similarly, and reproduction output itself is also liable to lower.
Further, an extremely fine ferromagnetic powder is necessary for the magnetic recording medium preferably used in a magnetic recording and reproducing system having incorporated MR magnetic heads capable of high density recording at high data transfer speed. However, the finer the powders, the more difficult is the production of ferromagnetic powders which satisfy both electromagnetic characteristics and storage stability (weather-fastness).